1. Field of the Invention:
This invention relates to a method of manufacturing a fluorescent display apparatus, the display area of which locally varies in luminous color in response to input signal.
2. Description of the Prior Art:
A fluorescent display apparatus provided with a filamentary cathode and a plurality of anodes having a phosphor layer applied thereon is known. Electrons emitted from the cathode are directed to selectively impinge upon the anodes to excite them for emitting light, thereby effecting displays of letters, patterns, etc. The fluorescent display apparatus can be driven at a relatively low voltage and is low in power consumption, and accordingly it is widely used as display device in various electronic instruments such as electronic desk calculators and digital clocks, as a digital display device in various measuring instruments and wireless instruments, as level meters in audio instruments, as analog display device in speed meters for motor vehicles, etc.
Phosphors which are used to form the phosphor layer applied on the anode in the fluorescent display apparatus must have a property such that the phosphor can be excited by electrons which have been accelerated by a relatively low anode voltage, so-called low speed electron beam, to emit light with sufficient luminescence for display, and ZnO:Zn system phosphors have generally been used.
The ZnO:Zn system phosphor is advantageous in that the value of the light emitting threshold voltage i.e., the, so-called dead voltage, is as low as the order of 1 to 2 V, and that, in ordinary display apparata of small-and-medium-types, sufficient luminescence for display can be obtained by an anode voltage of 20 V or less. The luminescence spectrum is in the vicinity of 504 nm, thus green light is obtained which is easy to see.
With the expanded application of the fluorescent display apparatus, a fluorescent display apparatus which can display luminous color other than green is required, depending upon the particular application and the nature of the input signal to be displayed.
In answer to such requirements, phosphors which can emit luminous color other than green have been developed and come into actual use, such as, for example, phosphors which are made of Y.sub.2 O.sub.3 or Y.sub.2 O.sub.2 S activated with Eu and which contain an adequate quantity of In.sub.2 O.sub.3 of ZnO for reducing resistance and can emit red luminous color upon impingement of low speed electron beam; and phosphors made of ZnS activated with Ag and coated with Cu or containing an adequate quantity of In.sub.2 O.sub.3 or ZnO for reducing resistance and which emit blue luminous color upon impingement of low speed electron beam.
With the advent of such various phosphors which can emit a luminous color other than green, the fluorescent display apparatus, which can display any desired colors in accordance with the environment and the application, has been manufactured. Further, there is a demand for a fluorescent display apparatus which can emit different luminous colors from different regions within the apparatus. Such an apparatus will be useful for a digital display apparatus because it can increase amount of information to be displayed or make reading of information to be displayed easier. In addition, such an apparatus can be applied to an analog display apparatus for indicating a warning signal or permitting direct observation of value of shift exceeding a set value.
When manufacturing the fluorescent display apparatus which includes two or more display regions, displaying different luminous colors from one another, the following problems are encountered.
One of the problems is that it is technically very difficult to apply, with one step of application process, two or more sorts of phosphors having different luminous colors with respect to one another on anode conductors which have been formed in different sections on an anode substrate. Accordingly, at present, as a rule, the respective sorts of phosphors having different luminous colors are applied to the anode conductors disposed in different sections by repeating a coating step two or more times.
In this case, however, if the coating of the phosphor is applied with the use of printing technique, a phosphor layer which has been printed in a first step can be crushed in the following second step by pressure by the printing screen, which results in a decrease of luminous efficiency.
Further, in the second printing step, the printing face of screen applies pressure on the prior printed phosphor layer, and accordingly the printing face can be stained with the phosphor of this prior printed phosphor layer. This staining phosphor can, in the second step, locally mix with the second printed phosphor layer, resulting in an unsightly luminescence of the second phosphor layer.
On the other hand, when the phosphor is applied using an electrodeposition technique, some of the phosphors which have been deposited in a first electrodeposition step will, in the following second electrodeposition step, attach on the surface of the second deposited phosphor layer thereby staining it. When the anode substrate, which has been fabricated through these two steps of electrodeposition, is used to manufacture the fluorescent display apparatus, the phosphor layer which has been locally stained with the other sort of phosphor, emit upon the impingement of electrons, respective luminous colors corresponding to the various phosphers from the anodes so stained. Thus a display of predetermined colors is unobtainable, and the display realized is unsightly.